Loop impedance
tests
19
a low test current is more susceptible to interference. This current
carries the advantage of reducing the risk of the RCD tripping but
carries the disadvantage that it produces weaker signals for the
measurement circuits to process.
8
If the instrument measures greater than 20
Ω
the over-range symbol
‘OL’ will be displayed. If this is the case, switch the instrument up a
range to the 200
Ω
range and repeat the test to obtain a satisfactory
reading. If on the 200
Ω
range the over-range symbol is again displayed,
switch the instrument up a range to the 2000
Ω
range and repeat the
test.
Note: Do not connect phase to phase as this instrument is rated at 230V.
9
External Earth Probe: the phase-earth loop impedance of exposed
metalwork (e.g. pipes/conduit ) can be tested using the external earth
probe supplied with the instrument. Connect the unit to the socket as
normal ensuring that the test button is not pressed or locked down.
Plug the external earth probe into the external earth probe socket,
ensuring the probe is held with fingers behind the finger guard. The red
LED will switch on as well as the two green LEDs remaining on. The
digital display will change to ‘LO’. Initially the buzzer will sound
indicating that the probe is plugged in. When this probe is connected
to an earth point, the red LED will switch off and the display will revert
to voltage mode. If the wiring status LEDs (two green LEDs) indicate
correct wiring, press the test button. A test will be initiated using the
external earth probe as the earth return path (the earth of the mains
lead will not be part of the circuit). During the test the symbols below
will be displayed indicating that a test is in progress. They are different
to the previous symbols for a normal earth test to indicate the probe
is being used. When the symbols stop, the measurement value will be
displayed.
6.5 Loop impedance at 3 phase equipment
Use the same procedure as in 6.4 above ensuring that only one phase is
connected at a time i.e.:
First Test: red prod to phase 1, black prod to neutral,
green crocodile clip to earth.
Second Test: red prod to phase 2, black prod to
neutral, green crocodile clip to earth etc.
